Exercise 22, § 5

Let $n$ be a natural number and let $K_n$ be a complete graph whose set of vertices is of cardinal $n($II, p. 220, Exerc. 5).

a) Exercise prove that the graphs $K_n$, for $1\leqslant n\leqslant 4$, are planar.

b) Exercise prove that the graph $K_5$ is not planar.